Different processes for the production of frozen food products are currently known, the main objective of which is that the product preserves its original organoleptic properties after it is thawed.
Most of the known processes cannot assure the preservation of the structure of the fruit and of some vegetables when they are thawed, mainly due to the large amount of water contained therein. Fruits normally contain more than 90% of water by weight.
With reference to very small-sized fruit, such as wild berries, although slow convection freezing systems (nitrogen vapors, CO2 or very cold air) are used, the freezing of the fruit occurs rapidly since its grains are small in size, therefore there is not enough time for the water to come out of the cell walls. The immediate consequence is that when these fruits thaw they relatively maintain their initial structure.
With reference to other fruits, convection freezing causes a freezing that is too slow, therefore the water freezes forming large crystals which break the structure of the fruit. When the fruit subsequently thaws, the water comes out of it, leaving the fruit with an appalling texture.
If the freezing is carried out by convection in liquids, i.e., immersing the product inside a bath with liquid nitrogen (at about −200° C.), liquid carbonic anhydride (at about −80° C.) or an aqueous solution optionally stirred at a very low temperature (between −50° and −35° C.), the freezing speed of the products is much quicker and the water freezes in the form of microscopic crystals which do not come out of the cells. The texture of the food consequently does not change and when the product thaws it maintains the original color, taste and texture.
The more rapidly a product has been frozen, the less liquid it releases upon thawing. At domestic level it is known that meat or fish must be left to thaw on a plate, so that the released liquid does not dirty the kitchen marble. A cause of this can be that it has been frozen by air convection, or that the freezing has been slow and the structure of said foods has been slightly broken. The lower the percentage of water the food has, the lower is the effect described.
Another reason producing liquid when the products thaw is that the cold chain has been broken at some time, the products being frozen again poorly. In many freezers of specialized stores, bags of frozen products with frost can be seen, which indicates a possible temporary breakage of the cold chain.
Another problem of freezing is that when water freezes its volume increases, therefore if a food with high water content is frozen, such as ripe fruit, the volume of the latter increases, many cracks in the surface or internal breakages occurring which cause irreparable damage in the food, losing its good appearance after thawing.
To solve these drawbacks, patent document US 2004/096559 describes a process for freezing vegetables or fruits, in which the products are partially dehydrated to considerably reduce the original water content of the product. According to this process, the products are dehydrated by means of hot air at a temperature greater than 90°, the water content preferably being reduced by between 25% and 60%. They are subsequently subjected to an ultrarapid freezing treatment, immersing the products in a bath of liquid nitrogen or of liquid carbon dioxide at a temperature close to −200° C. Upon thawing, the products maintain their taste, but not their appearance, since a very considerable part of their water has been removed from them.
For the purpose of reducing the size of the ice crystals produced inside the fruits subjected to a freezing process, patent document EP 1525801 describes a process in which the fruit is subjected to a regime of temperature reduction in stages. The fruit is slowly cooled to 0° C., it is subsequently slowly under-cooled to a temperature between −8° C. and −12° C. at a cooling rate from 10° C. to more than 40° C. per hour, so that the temperature difference between the surface and the inside is 1.5° C.; and finally it is additionally cooled until ice formation occurs. Although this process achieves preserving the taste of the fruit better than other alternative processes, in which the fruit is sprayed with sugary powders or fluids, it is aimed at the fruits being consumed in their frozen state. In the event that the fruits are thawed, they considerably lose their original texture.
Patent documents FR 2878412, EP 0572745 and U.S. Pat. No. 6,004,607 describe a process in which the products are immersed in an aqueous solution for a superficial pre-freezing, before being cooled or frozen using conventional methods. According to these documents, upon immersing the products in the aqueous solution the freezing of the envelopment of the fruit, of its outermost layer, occurs, without the freezing of the core of the fruit occurring for the moment, which reduces the damage subsequently caused in the envelopment of the product when ice crystals are formed therein. These processes have, nevertheless, an important drawback, which is the change of taste conferred to the immersed products by the products added to the aqueous solution to decrease the freezing point of the water. The products usually used are sodium chloride, calcium chloride, sugars or alcohols.
An objective of the present invention is to disclose a quick and cost-effective process for the production of frozen foods, in which the breakage of the internal structure of the foods is not prevented, so that once thawed their appearance, taste, odor and texture are similar to those which they had before they were frozen.